This is a multicenter study of over 200 patients, at least 20 of whom are hoped to be studies at Boston City Hospital. Once patients have met eligibility criteria they will be assigned to a one wk single blind placebo to ensure that they remain in atrial fibrillation. They will then receive one wk of double-blind placebo or D-sotalol (200 mg twice daily). If at the end of that time, they remain in atrial fibrillation they will undergo electrical cardioversion and will be followed up for the subsequent 6 months.